The present invention relates to a process and apparatus for applying orienting layers to a substrate for alignment of liquid crystal molecules, especially for large-scale liquid crystal displays by sputtering materials which tend to grow prismatic crystals on the substrate by plasma deposition.
The orientation of liquid crystal molecules of a display cell in a predetermined direction is crucial for operation of predominantly liquid crystal displays, especially of twisted nematic liquid crystal displays(TN-LCDs) or also ferroelectric liquid crystal displays(FLCDs). Because of this orientation of the "liquid crystals" their optical anisotropy can produce a macroscopically observable effect. This orientation generally occurs by application of an anisotropically orienting layer to the inner surfaces of each display substrate, before it is glued to a display comprising a covering and base substrate. Subsequently the liquid crystal is filled into the display. The orienting layers cause a recrystallization of the liquid crystal layers in a predetermined alignment.
Several different processes are already known for making these orienting layers, of which the most important are described here briefly with their advantages and disadvantages. SiO.sub.2, for example from an electron beam vapor source, can be deposited as an orienting layer in a vacuum chamber on the inner substrate surface of a display at a very small angle of inclination of about 5.degree. to the substrate surface. During growth of the SiO.sub.2 layer prismatic crystals obliquely inclined in the direction of the vapor source on whose surfaces the liquid crystal molecules are deposited are produced. Because of the strongly anisotropic orientation of the oblique prismatic crystals a very impressive and homogeneous orientation results. The inclination angle of the prismatic crystals can be influenced to a certain extent by the vapor deposition parameters and thus also the so-called "edge tilt angle" of the liquid crystal molecules can be varied, which has a decisive effect on the switching properties of the display. Further, the manufacturing process for the orienting layers and thus the orientation properties are very reproducible by this vapor deposition. Furthermore, the orientation of the liquid crystals is very uniform. The SiO.sub.2 -layer is chemically very stable and sensitive to UV-light and high temperatures. Also the liquid crystal does not combine chemically with the SiO.sub.2 layer and can not change, i.e. no or minor and extinguishable formation of transient images occurs (image sticking). Because of the comparatively large surface area and hydrophilic character of the SiO.sub.2 -prismatic crystals, on introduction of the liquid crystals into the display frequently a chromatic separation of the individual components of the liquid crystal mixture occurs, whereby the switching behavior of the display in the separated regions is changed. An additional disadvantage of this process is that the vapor deposition sources are usually point sources and thus deposition on a comparatively large surface in a uniform manner is not possible. The alignment of the prismatic crystals varies in the individual substrate regions, since the molecules issue or impinge on the substrate surface at different angles from the point source. The inhomogeneity of course can be reduced in one direction by arranging several vapor deposition sources in a line. Because of that, however, the vapor deposition performance of the individual sources must be carefully accurately regulated. Moreover a very large apparatus expense, and especially a comparatively large vacuum processing chamber with associated large scale pumps and air locks, are required, which can only be provided by a very costly special manufacturing process. These processes have only a very limited applicability to the making of large scale displays.
The most widely used methods for making of the orienting layers in the industrial manufacture of liquid crystal displays utilize organic layers such as polyimides, polyvinylalcohols or other plastic films which are flung or centrifuged on the substrates in a liquid phase. After a suitable drying and hardening process this plastic film which is a few nanometers thick is rubbed or ground with a carbon fiber or velvet brush in one direction. Because of that, microscopic comparatively small tracks or tears are produced in the layer and an orientation of the organic molecules and/or molecular chains occurs. The resulting micro-mechanical and/or molecular anisotropy of the film causes an orientation of the liquid crystals later filled into the display. The advantage of this process is that the size of the substrate surface plays no role or has no effect. This process is also comparatively economical; however it is only reproducible to a limited extent. The result of the orientation of the molecules depends on a very large number of process parameters such as the rubbing pressure of the velvet or carbon fiber brushes, rubbing strength, polymerization, and crystallization degree of the plastic film and its chemical properties as well as the surface reactions of the film with water from the air or from solvents. These process parameters can be optimized only with difficulty. Furthermore, the plastic layers are only comparatively poorly stable relative to temperature changes or UV light. With ferroelectric liquid crystals there is also the disadvantage that the organic molecules of the orienting film combine vigorously chemically with the organic liquid crystal molecules and thus are rotated with the liquid crystal molecules on application of an electric field or hold them in their position and thus make the required rotation of molecules difficult. The result is a so-called "burning-in" of written-in or stored images ("image sticking") or a monostable switching behavior of the normally bistable switching ferroelectric liquid crystal.
Besides the brushing or rubbing of plastic film an orientation of polymer films with the aid of linearly polarized UV-light is also known. The polymerization is direction-dependent so as to provide the required anisotropy because of the direction-dependence of the polarized light. The production of microstructure in a plastic layer by impression or stamping techniques or photolithography has already been tried. The disadvantage of comparatively poor long-term stability of the plastic layers relative to temperature changes and UV light exposure and the chemical reactivity of the plastic molecules with the liquid crystal molecules however cannot be eliminated by this process.
Also, already there have been sporadic suggestions regarding the possibility of an inclined sputtering or deposition of material with low pressure plasmas, which lead to prismatic growth. Aluminum nitride is a particularly suitable material for this type of deposition and leads to prismatic growth in a nitrogen atmosphere when the deposition is at comparatively large impingement angles to the substrate surface. However, when a comparatively large substrate surface must be sputtered from a point-like target difficulties which are similar to those encountered with vapor deposition result. Indeed large-surface area targets can be used here, whereby comparatively fewer inhomogeneities result because the sputtering processes are more diffuse in comparison to vapor deposition. However, these methods also simultaneously reduce the directional anisotropy required for the orientation of the prismatic crystals.